(1) Field of Invention
The present invention relates to phase comparison between a signal and a reference with irregular sampling of the phase and interpolation before regular resampling.
(2) Description of the Related Art
Numerical phase detectors assign numerical values to the time of occurrence (phase) of significant instants (zero crossings, edges, or arrival times) of an input signal compared to a reference signal. These phase detectors are used in phase-measurement instruments when high-frequency jitter is to be measured. In particular, time-interval analyzers apply a numerical time-stamp to signal edges, and some jitter measurement instruments measure the position of each zero crossing (see U.S. Pat. Nos. 6,255,866 and 6,529,842). Numerical phase detectors are also used in phase-locked loops for the purpose of synchronizing the frequency and phase of two signals. In particular, phase-locked loops that include a FIFO for data and phase-locked loops that have a very low bandwidth (milliHertz) use a numerical phase detector.
Numerical phase detectors can provide phase information only at the time of the event or edge. Therefore the timing of the phase information is irregular if the edges are irregularly spaced. In fact, any phase modulation on the input signal assures the intervals will be irregular, with the irregularity increasing for strong modulation. When the input signal is a Non-Return-to-Zero (NRZ) data signal, the edge spacing (and, consequently, the phase sample spacing) is very irregular—often varying over a range of ten to one. If the phase information is to be processed by filtering or rms calculations, the irregular timing introduces an error or makes the processing difficult.
In prior art, the problem of irregular phase-sample spacing is dealt with by simply resampling at regular intervals (see FIG. 1). But some phase values are missed when their spacing is shorter than the resampling interval, and other values are repeated when their spacing is longer than the resampling interval. This distorts the original waveform of the phase and leads to inaccurate results when the phase is filtered. An example of this waveform distortion is shown in FIG. 6. Here the INPUT signal is NRZ data, and the phase is ramping smoothly upward. It can be seen that the longer intervals between input edges cause repeated values in the resampled phase N3.
The problem of waveform distortion could be solved by performing interpolation before the resampling (see FIG. 7). The resampled phase would then be smooth, as shown in FIG. 8. Some simple interpolation schemes are know that apply to interpolation over regularly spaced intervals; U.S. Pat. No. 6,255,866 uses such a scheme in preparing phase information for jitter generation. The need here is to realize an interpolator that deal with changing interpolation intervals at the same time as changing phase increments. This can be done with software, but the algorithm is slow and usually can't be used with real-time applications. The interpolation can be done at high-speed with large custom circuits, but the realization is expensive; a typical high-speed interpolator is described in U.S. Pat. No. 5,020,014.
Because of the size and the expense, interpolation has not been used in real-time phase measurement with variable interpolation intervals.